1866 as a shortening of professional (n.). The adjective is attested by 1915 (in golfing's pro shop, workshop run by the resident professional at a club). The use of professional in reference to prostitutes seems to have accounted for proette in sports writing for "female pro golfer" (1968).
"a consideration or argument in favor," c. 1400, from Latin pro (prep.) "on behalf of, in place of, before, for, in exchange for, just as" (from PIE root *per- (1) "forward," hence "in front of, before, first, chief"). Pro and con is short for pro and contra (c. 1400) "for and against" (Latin pro et contra).